| Current distributed multimedia systems, including the World Wide Web, give users access to vast amounts of media-rich information. Much effort has been put into providing browsing and querying tools, storing multimedia documents efficiently, and transporting the documents from a server to a client. The multimedia data in these systems is often protected by copyright and has other restrictions on its use. However, current systems, such as the Web, have limited support for document security.; When an authorised user obtains a document, its use is no longer under the scrutiny of the system--the document's provider has no control over how it is used. In this thesis we present a system, along with a sample implementation, designed to extend the system's control over documents. It determines precisely what the user may and may not do after the document has been transferred to the user's client machine. The basis of the system is the notion of a contract which describes the operations users may perform on the document, a precise representation of a license agreement between the document's creator and a user of the document. This contract is enforced by a tool which is dynamically produced, sent to the client, and used by the user to manipulate the document. |
